The interconversions of the various forms of the guinea pig and rabbit uterine progestin receptor may best be described as: 3S less than 4.5S less than greater than 5.5S less than greater than 7S less than greater than 8.5S less than greater than High Molecular weight forms. Our sedimentation data indicate that one function of the receptor (in addition ot stabilizing the 7S form) is to drive the equilibrium to the right. Our researches are designed to substantiate this universal model by identifying each of the subunits and characterizing their interconversions. Toward this goal we have developed a hybridoma cell line (KN 382/EC1) which produces an IgG1 directed against the 8.55 form of the receptor. With this new tool we have developed a two-prolonged attack on the problem which encompasses both a biochemical and an immunological approach. We shall utilize hydroxylapatite chromatography, DEAE chromatography, high performance liquid chromatography and electrophoresis (one and two dimensional) coupled with immunoaffinity chromatography, immunoprecipitation to isolate, identify and characterize the subunits. In addition we shall develop a second generation of hybridoma cell lines producing antibody directed against the recepter.